Method for terminal managing configuration for device-to-device (D2D) operation in wireless communication system, and terminal using same

ABSTRACT

Provided are a method for a terminal managing a configuration for a device-to-device (D2D) operation in a wireless communication system, and a terminal using the method. The method is characterized by receiving a D2D configuration for a D2D operation from a network, and maintaining the D2D configuration for the D2D operation without releasing same when leaving the radio resource control (RRC) connected state.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage filing under 35 U.S.C. 371 ofInternational Application No. PCT/KR2015/010314, filed on Sep. 30, 2015,which claims the benefit of U.S. Provisional Applications No. 62/056,482filed on Sep. 27, 2014, and No. 62/060,982 filed on Oct. 7, 2014, thecontents of which are all hereby incorporated by reference herein intheir entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a wireless communication system, moreparticularly a method of managing a configuration for a D2D operationperformed by a UE in the wireless communication system and the UE usingthe method.

Related Art

In International Telecommunication Union Radio communication sector(ITU-R), a standardization task for International MobileTelecommunication (IMT)-Advanced, that is, the next-generation mobilecommunication system since the third generation, is in progress.IMT-Advanced sets its goal to support Internet Protocol (IP)-basedmultimedia services at a data transfer rate of 1 Gbps in the stop andslow-speed moving state and of 100 Mbps in the fast-speed moving state.

For example, 3^(rd) Generation Partnership Project (3GPP) is a systemstandard to satisfy the requirements of IMT-Advanced and is preparingfor LTE-Advanced improved from Long Term Evolution (LTE) based onOrthogonal Frequency Division Multiple Access (OFDMA)/SingleCarrier-Frequency Division Multiple Access (SC-FDMA) transmissionschemes. LTE-Advanced is one of strong candidates for IMT-Advanced.

There is a growing interest in a Device-to-Device (D22) technology inwhich devices perform direct communication. In particular, D2D has beenin the spotlight as a communication technology for a public safetynetwork. A commercial communication network is rapidly changing to LTE,but the current public safety network is basically based on the 2Gtechnology in terms of a collision problem with existing communicationstandards and a cost. Such a technology gap and a need for improvedservices are leading to efforts to improve the public safety network.

The public safety network has higher service requirements (reliabilityand security) than the commercial communication network. In particular,if coverage of cellular communication is not affected or available, thepublic safety network also requires direct communication betweendevices, that is, D2D operation.

D2D operation may have various advantages in that it is communicationbetween devices in proximity. For example, D2D UE has a high transferrate and a low delay and may perform data communication. Furthermore, inD2D operation, traffic concentrated on a base station can bedistributed. If D2D UE plays the role of a relay, it may also play therole of extending coverage of a base station.

Meanwhile, the UE may be in an RRC connection state having a radioresource control (RRC) connection with the network, and may leave theRRC connection state for various reasons. For example, if the networkreleases an RRC connection or attempts to reestablish an RRC connectiondue to poor state of a wireless link with the network, then the UE mayleave the RRC connection state.

According to prior art, when the UE leaves the RRC connection state, theUE releases all the radio resources that have been configured to the UE.For example, the UE releases a media access control (MAC) configurationfor the established radio bearers and the associated Packet DataConvergence Protocol (PDCP) entity and releases the radio link control(RLC) entity.

According to such prior art, when the UE leaves the RRC connectionstate, the configuration for the D2D operation will also be released. Asa result, the continuity of the D2D operation may not be guaranteed forthe UE leaving the RRC connection state.

SUMMARY OF THE INVENTION

Technical aspect which is to be solved in the present invention is toprovide a method of managing a configuration on a D2D operationperformed by a UE in a wireless communication system and the UE usingthe method.

In one aspect, provided is a method for managing a configuration fordevice-to-device (D2D) operation of a user equipment (UE) in a wirelesscommunication system. The method comprising receiving a D2Dconfiguration on the D2D operation from a network and maintaining,without releasing, the D2D configuration on the D2D operation, whenleaving a RRC (radio resource control) connection state.

The method may further comprise determining whether a condition totransit from the RRC connection state to another state is satisfied, andwherein the condition is characterized in that the network releases theRRC connection on the UE, or the UE fails a RRC connectionreconfiguration with the network.

The D2D operation may be performed by applying the maintained D2Dconfiguration until the UE selects a specific cell and receives systeminformation on a D2D operation from the selected specific cell.

A cell which does not provide system information on a D2D operation maybe barred, in selecting the specific cell.

The specific cell may be selected from cells which provide systeminformation on a D2D operation.

When the UE leaves a RRC connection state, a timer may be started, andthe UE may perform a D2D operation by applying the D2D configurationonly for a time duration indicated by the timer.

The network may provide an additional D2D configuration other than theD2D configuration, and the additional D2D configuration may be availableto use after the UE releases a RRC connection state.

The D2D operation may be a D2D communication.

In another aspect, provided is a user equipment (UE). The UE includes aradio frequency (RF) unit configured to transmit and receive a wirelesssignal and a processor operatively connected to the RF unit andconfigured to be operated. The processor further configured to: receivea D2D configuration on a D2D operation from a network; and maintain,without releasing, the D2D configuration on the D2D operation, whenleaving a RRC (radio resource control) connection state.

According to the present invention, continuity of the D2D operation maybe guaranteed even if the UE leaves the RRC connection state whileperforming the D2D operation. Reliability is very important when D2Doperation is used for public safety, and the present invention mayincrease the reliability of such D2D operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wireless communication system to which the presentinvention is applied.

FIG. 2 is a diagram showing a wireless protocol architecture for a userplane.

FIG. 3 is a diagram showing a wireless protocol architecture for acontrol plane.

FIG. 4 is a flowchart illustrating the operation of UE in the RRC idlestate.

FIG. 5 is a flowchart illustrating a process of establishing RRCconnection.

FIG. 6 is a flowchart illustrating an RRC connection reconfigurationprocess.

FIG. 7 is a diagram illustrating an RRC connection re-establishmentprocedure.

FIG. 8 illustrates substates which may be owned by UE in the RRC_IDLEstate and a substate transition process.

FIG. 9 shows a basic structure for ProSe.

FIG. 10 shows the deployment examples of types of UE performing ProSedirect communication and cell coverage.

FIG. 11 shows a user plane protocol stack for ProSe directcommunication.

FIG. 12 shows the PC 5 interface for D2D direct discovery.

FIG. 13 is an embodiment of a ProSe discovery process.

FIG. 14 is another embodiment of a ProSe discovery process.

FIG. 15 shows a method of managing a configuration on the D2D operationof UE according to an embodiment of the present invention.

FIG. 16 shows a case where the RRC connection re-establishment procedureis successful.

FIG. 17 shows a case where the RRC connection re-establishment procedurefails.

FIG. 18 shows an example of a serving frequency and a frequency forperforming D2D operation on a UE-by-UE basis.

FIG. 19 illustrates a cell selection method of a UE according to anembodiment of the present invention.

FIG. 20 is a block diagram showing a UE in which an embodiment of thepresent invention is implemented.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 shows a wireless communication system to which the presentinvention is applied. The wireless communication system may also bereferred to as an evolved-UMTS terrestrial radio access network(E-UTRAN) or a long term evolution (LTE)/LTE-A system.

The E-UTRAN includes at least one base station (BS) 20 which provides acontrol plane and a user plane to a user equipment (UE) 10. The UE 10may be fixed or mobile, and may be referred to as another terminology,such as a mobile station (MS), a user terminal (UT), a subscriberstation (SS), a mobile terminal (MT), a wireless device, etc. The BS 20is generally a fixed station that communicates with the UE 10 and may bereferred to as another terminology, such as an evolved node-B (eNB), abase transceiver system (BTS), an access point, etc.

The BSs 20 are interconnected by means of an X2 interface. The BSs 20are also connected by means of an S1 interface to an evolved packet core(EPC) 30, more specifically, to a mobility management entity (MME)through S1-MME and to a serving gateway (S-GW) through S1-U.

The EPC 30 includes an MME, an S-GW, and a packet data network-gateway(P-GW). The MME has access information of the UE or capabilityinformation of the UE, and such information is generally used formobility management of the UE. The S-GW is a gateway having an E-UTRANas an end point. The P-GW is a gateway having a PDN as an end point.

Layers of a radio interface protocol between the UE and the network canbe classified into a first layer (L1), a second layer (L2), and a thirdlayer (L3) based on the lower three layers of the open systeminterconnection (OSI) model that is well-known in the communicationsystem. Among them, a physical (PHY) layer belonging to the first layerprovides an information transfer service by using a physical channel,and a radio resource control (RRC) layer belonging to the third layerserves to control a radio resource between the UE and the network. Forthis, the RRC layer exchanges an RRC message between the UE and the BS.

FIG. 2 is a diagram showing a wireless protocol architecture for a userplane. FIG. 3 is a diagram showing a wireless protocol architecture fora control plane. The user plane is a protocol stack for user datatransmission. The control plane is a protocol stack for control signaltransmission.

Referring to FIGS. 2 and 3, a PHY layer provides an upper layer with aninformation transfer service through a physical channel. The PHY layeris connected to a medium access control (MAC) layer which is an upperlayer of the PHY layer through a transport channel Data is transferredbetween the MAC layer and the PHY layer through the transport channel.The transport channel is classified according to how and with whatcharacteristics data is transferred through a radio interface.

Data is moved between different PHY layers, that is, the PHY layers of atransmitter and a receiver, through a physical channel. The physicalchannel may be modulated according to an Orthogonal Frequency DivisionMultiplexing (OFDM) scheme, and use the time and frequency as radioresources.

The functions of the MAC layer include mapping between a logical channeland a transport channel and multiplexing and demultiplexing to atransport block that is provided through a physical channel on thetransport channel of a MAC Service Data Unit (SDU) that belongs to alogical channel. The MAC layer provides service to a Radio Link Control(RLC) layer through the logical channel.

The functions of the RLC layer include the concatenation, segmentation,and reassembly of an RLC SDU. In order to guarantee various types ofQuality of Service (QoS) required by a Radio Bearer (RB), the RLC layerprovides three types of operation mode: Transparent Mode (TM),Unacknowledged Mode (UM), and Acknowledged Mode (AM). AM RLC provideserror correction through an Automatic Repeat Request (ARQ).

The RRC layer is defined only on the control plane. The RRC layer isrelated to the configuration, reconfiguration, and release of radiobearers, and is responsible for control of logical channels, transportchannels, and PHY channels. An RB means a logical route that is providedby the first layer (PHY layer) and the second layers (MAC layer, the RLClayer, and the PDCP layer) in order to transfer data between UE and anetwork.

The function of a Packet Data Convergence Protocol (PDCP) layer on theuser plane includes the transfer of user data and header compression andciphering. The function of the PDCP layer on the user plane furtherincludes the transfer and encryption/integrity protection of controlplane data.

What an RB is configured means a process of defining the characteristicsof a wireless protocol layer and channels in order to provide specificservice and configuring each detailed parameter and operating method. AnRB can be divided into two types of a Signaling RB (SRB) and a Data RB(DRB). The SRB is used as a passage through which an RRC message istransmitted on the control plane, and the DRB is used as a passagethrough which user data is transmitted on the user plane.

If RRC connection is established between the RRC layer of UE and the RRClayer of an E-UTRAN, the UE is in the RRC connected state. If not, theUE is in the RRC idle state.

A downlink transport channel through which data is transmitted from anetwork to UE includes a broadcast channel (BCH) through which systeminformation is transmitted and a downlink shared channel (SCH) throughwhich user traffic or control messages are transmitted. Traffic or acontrol message for downlink multicast or broadcast service may betransmitted through the downlink SCH, or may be transmitted through anadditional downlink multicast channel (MCH). Meanwhile, an uplinktransport channel through which data is transmitted from UE to a networkincludes a random access channel (RACH) through which an initial controlmessage is transmitted and an uplink shared channel (SCH) through whichuser traffic or control messages are transmitted.

Logical channels that are placed over the transport channel and that aremapped to the transport channel include a broadcast control channel(BCCH), a paging control channel (PCCH), a common control channel(CCCH), a multicast control channel (MCCH), and a multicast trafficchannel (MTCH).

The physical channel includes several OFDM symbols in the time domainand several subcarriers in the frequency domain One subframe includes aplurality of OFDM symbols in the time domain. An RB is a resourcesallocation unit, and includes a plurality of OFDM symbols and aplurality of subcarriers. Furthermore, each subframe may use specificsubcarriers of specific OFDM symbols (e.g., the first OFDM symbol) ofthe corresponding subframe for a physical downlink control channel(PDCCH), that is, an L1/L2 control channel. A Transmission Time Interval(TTI) is a unit time for subframe transmission.

The RRC state of UE and an RRC connection method are described below.

The RRC state means whether or not the RRC layer of UE is logicallyconnected to the RRC layer of the E-UTRAN. A case where the RRC layer ofUE is logically connected to the RRC layer of the E-UTRAN is referred toas an RRC connected state. A case where the RRC layer of UE is notlogically connected to the RRC layer of the E-UTRAN is referred to as anRRC idle state. The E-UTRAN may check the existence of corresponding UEin the RRC connected state in each cell because the UE has RRCconnection, so the UE may be effectively controlled. In contrast, theE-UTRAN is unable to check UE in the RRC idle state, and a Core Network(CN) manages UE in the RRC idle state in each tracking area, that is,the unit of an area greater than a cell. That is, the existence ornon-existence of UE in the RRC idle state is checked only for each largearea. Accordingly, the UE needs to shift to the RRC connected state inorder to be provided with common mobile communication service, such asvoice or data.

When a user first powers UE, the UE first searches for a proper cell andremains in the RRC idle state in the corresponding cell. The UE in theRRC idle state establishes RRC connection with an E-UTRAN through an RRCconnection procedure when it is necessary to set up the RRC connection,and shifts to the RRC connected state. A case where UE in the RRC idlestate needs to set up RRC connection includes several cases. Forexample, the cases may include a need to send uplink data for a reason,such as a call attempt by a user, and to send a response message as aresponse to a paging message received from an E-UTRAN.

A Non-Access Stratum (NAS) layer placed over the RRC layer performsfunctions, such as session management and mobility management.

In the NAS layer, in order to manage the mobility of UE, two types ofstates: EPS Mobility Management-REGISTERED (EMM-REGISTERED) andEMM-DEREGISTERED are defined. The two states are applied to UE and theMME. UE is initially in the EMM-DEREGISTERED state. In order to access anetwork, the UE performs a process of registering it with thecorresponding network through an initial attach procedure. If the attachprocedure is successfully performed, the UE and the MME become theEMM-REGISTERED state.

In order to manage signaling connection between UE and the EPC, twotypes of states: an EPS Connection Management (ECM)-IDLE state and anECM-CONNECTED state are defined. The two states are applied to UE andthe MME. When the UE in the ECM-IDLE state establishes RRC connectionwith the E-UTRAN, the UE becomes the ECM-CONNECTED state. The MME in theECM-IDLE state becomes the ECM-CONNECTED state when it establishes S1connection with the E-UTRAN. When the UE is in the ECM-IDLE state, theE-UTRAN does not have information about the context of the UE.Accordingly, the UE in the ECM-IDLE state performs procedures related toUE-based mobility, such as cell selection or cell reselection, without aneed to receive a command from a network. In contrast, when the UE is inthe ECM-CONNECTED state, the mobility of the UE is managed in responseto a command from a network. If the location of the UE in the ECM-IDLEstate is different from a location known to the network, the UE informsthe network of its corresponding location through a tracking area updateprocedure.

System information is described below.

System information includes essential information that needs to be knownby UE in order for the UE to access a BS. Accordingly, the UE needs tohave received all pieces of system information before accessing the BS,and needs to always have the up-to-date system information. Furthermore,the BS periodically transmits the system information because the systeminformation is information that needs to be known by all UEs within onecell. The system information is divided into a Master Information Block(MIB) and a plurality of System Information Blocks (SIBs).

The MIB may include the limited number of parameters which are the mostessential and are most frequently transmitted in order to obtain otherinformation from a cell. UE first discovers an MIB after downlinksynchronization. The MIB may include information, such as a downlinkchannel bandwidth, a PHICH configuration, an SFN supportingsynchronization and operating as a timing reference, and an eNBtransmission antenna configuration. The MIB may be broadcasted on a BCH.SystemInformationBlockType1 (SIB1) of included SIBs is included in a“SystemInformationBlockType1” message and transmitted. Other SIBs otherthan the SIB1 are included in a system information message andtransmitted. The mapping of the SIBs to the system information messagemay be flexibly configured by a scheduling information list parameterincluded in the SIB1. In this case, each SIB is included in a singlesystem information message. Only SIBs having the same schedulingrequired value (e.g. period) may be mapped to the same systeminformation message. Furthermore, SystemInformationBlockType2 (SIB2) isalways mapped to a system information message corresponding to the firstentry within the system information message list of a schedulinginformation list. A plurality of system information messages may betransmitted within the same period. The SIB1 and all of the systeminformation messages are transmitted on a DL-SCH.

In addition to broadcast transmission, in the E-UTRAN, the SIB1 may bechannel-dedicated signaling including a parameter set to have the samevalue as an existing set value. In this case, the SIB1 may be includedin an RRC connection re-establishment message and transmitted.

The SIB1 includes information related to UE cell access and defines thescheduling of other SIBs. The SIB1 may include information related tothe PLMN identifiers, Tracking Area Code (TAC), and cell ID of anetwork, a cell barring state indicative of whether a cell is a cell onwhich UE can camp, a required minimum reception level within a cellwhich is used as a cell reselection reference, and the transmission timeand period of other SIBs.

The SIB2 may include radio resource configuration information common toall types of UE. The SIB2 may include information related to an uplinkcarrier frequency and uplink channel bandwidth, an RACH configuration, apage configuration, an uplink power control configuration, a soundingreference signal configuration, a PUCCH configuration supportingACK/NACK transmission, and a PUSCH configuration.

UE may apply a procedure for obtaining system information and fordetecting a change of system information to only a PCell. In an SCell,when the corresponding SCell is added, the E-UTRAN may provide all typesof system information related to an RRC connection state operationthrough dedicated signaling. When system information related to aconfigured SCell is changed, the E-UTRAN may release a considered SCelland add the considered SCell later. This may be performed along with asingle RRC connection re-establishment message. The E-UTRAN may set avalue broadcast within a considered SCell and other parameter valuethrough dedicated signaling.

UE needs to guarantee the validity of a specific type of systeminformation. Such system information is called required systeminformation. The required system information may be defined as follows.

-   -   If UE is in the RRC_IDLE state: the UE needs to have the valid        version of the MIB and the SIB1 in addition to the SIB2 to SIB8.        This may comply with the support of a considered RAT.    -   If UE is in the RRC connection state: the UE needs to have the        valid version of the MIB, SIB1, and SIB2.

In general, the validity of system information may be guaranteed up to amaximum of 3 hours after being obtained.

In general, service that is provided to UE by a network may beclassified into three types as follows. Furthermore, the UE differentlyrecognizes the type of cell depending on what service may be provided tothe UE. In the following description, a service type is first described,and the type of cell is described.

1) Limited service: this service provides emergency calls and anEarthquake and Tsunami Warning System (ETWS), and may be provided by anacceptable cell.

2) Suitable service: this service means public service for common uses,and may be provided by a suitable cell (or a normal cell).

3) Operator service: this service means service for communicationnetwork operators. This cell may be used by only communication networkoperators, but may not be used by common users.

In relation to a service type provided by a cell, the type of cell maybe classified as follows.

1) An acceptable cell: this cell is a cell from which UE may be providedwith limited service. This cell is a cell that has not been barred froma viewpoint of corresponding UE and that satisfies the cell selectioncriterion of the UE.

2) A suitable cell: this cell is a cell from which UE may be providedwith suitable service. This cell satisfies the conditions of anacceptable cell and also satisfies additional conditions. The additionalconditions include that the suitable cell needs to belong to a PublicLand Mobile Network (PLMN) to which corresponding UE may access and thatthe suitable cell is a cell on which the execution of a tracking areaupdate procedure by the UE is not barred. If a corresponding cell is aCSG cell, the cell needs to be a cell to which UE may access as a memberof the CSG.

3) A barred cell: this cell is a cell that broadcasts informationindicative of a barred cell through system information.

4) A reserved cell: this cell is a cell that broadcasts informationindicative of a reserved cell through system information.

FIG. 4 is a flowchart illustrating the operation of UE in the RRC idlestate. FIG. 4 illustrates a procedure in which UE that is initiallypowered on experiences a cell selection process, registers it with anetwork, and then performs cell reselection if necessary.

Referring to FIG. 4, the UE selects Radio Access Technology (RAT) inwhich the UE communicates with a Public Land Mobile Network (PLMN), thatis, a network from which the UE is provided with service (S410).Information about the PLMN and the RAT may be selected by the user ofthe UE, and the information stored in a Universal Subscriber IdentityModule (USIM) may be used.

The UE selects a cell that has the greatest value and that belongs tocells having measured BS and signal intensity or quality greater than aspecific value (cell selection) (S420). In this case, the UE that ispowered off performs cell selection, which may be called initial cellselection. A cell selection procedure is described later in detail.After the cell selection, the UE receives system informationperiodically by the BS. The specific value refers to a value that isdefined in a system in order for the quality of a physical signal indata transmission/reception to be guaranteed. Accordingly, the specificvalue may differ depending on applied RAT.

If network registration is necessary, the UE performs a networkregistration procedure (S430). The UE registers its information (e.g.,an IMSI) with the network in order to receive service (e.g., paging)from the network. The UE does not register it with a network whenever itselects a cell, but registers it with a network when information aboutthe network (e.g., a Tracking Area Identity (TAI)) included in systeminformation is different from information about the network that isknown to the UE.

The UE performs cell reselection based on a service environment providedby the cell or the environment of the UE (S440). If the value of theintensity or quality of a signal measured based on a BS from which theUE is provided with service is lower than that measured based on a BS ofa neighboring cell, the UE selects a cell that belongs to other cellsand that provides better signal characteristics than the cell of the BSthat is accessed by the UE. This process is called cell reselectiondifferently from the initial cell selection of the No. 2 process. Inthis case, temporal restriction conditions are placed in order for acell to be frequently reselected in response to a change of signalcharacteristic. A cell reselection procedure is described later indetail.

FIG. 5 is a flowchart illustrating a process of establishing RRCconnection.

UE sends an RRC connection request message that requests RRC connectionto a network (S510). The network sends an RRC connection establishmentmessage as a response to the RRC connection request (S520). Afterreceiving the RRC connection establishment message, the UE enters RRCconnected mode.

The UE sends an RRC connection establishment complete message used tocheck the successful completion of the RRC connection to the network(S530).

FIG. 6 is a flowchart illustrating an RRC connection reconfigurationprocess. An RRC connection reconfiguration is used to modify RRCconnection. This is used to establish/modify/release RBs, performhandover, and set up/modify/release measurements.

A network sends an RRC connection reconfiguration message for modifyingRRC connection to UE (S610). As a response to the RRC connectionreconfiguration message, the UE sends an RRC connection reconfigurationcomplete message used to check the successful completion of the RRCconnection reconfiguration to the network (S620).

Hereinafter, a public land mobile network (PLMN) is described.

The PLMN is a network which is disposed and operated by a mobile networkoperator. Each mobile network operator operates one or more PLMNs. EachPLMN may be identified by a Mobile Country Code (MCC) and a MobileNetwork Code (MNC). PLMN information of a cell is included in systeminformation and broadcasted.

In PLMN selection, cell selection, and cell reselection, various typesof PLMNs may be considered by the terminal.

Home PLMN (HPLMN): PLMN having MCC and MNC matching with MCC and MNC ofa terminal IMSI.

Equivalent HPLMN (EHPLMN): PLMN serving as an equivalent of an HPLMN.

Registered PLMN (RPLMN): PLMN successfully finishing locationregistration.

Equivalent PLMN (EPLMN): PLMN serving as an equivalent of an RPLMN.

Each mobile service consumer subscribes in the HPLMN. When a generalservice is provided to the terminal through the HPLMN or the EHPLMN, theterminal is not in a roaming state. Meanwhile, when the service isprovided to the terminal through a PLMN except for the HPLMN/EHPLMN, theterminal is in the roaming state. In this case, the PLMN refers to aVisited PLMN (VPLMN).

When UE is initially powered on, the UE searches for available PublicLand Mobile Networks (PLMNs) and selects a proper PLMN from which the UEis able to be provided with service. The PLMN is a network that isdeployed or operated by a mobile network operator. Each mobile networkoperator operates one or more PLMNs. Each PLMN may be identified byMobile Country Code (MCC) and Mobile Network Code (MNC).

Information about the PLMN of a cell is included in system informationand broadcasted. The UE attempts to register it with the selected PLMN.If registration is successful, the selected PLMN becomes a RegisteredPLMN (RPLMN). The network may signalize a PLMN list to the UE. In thiscase, PLMNs included in the PLMN list may be considered to be PLMNs,such as RPLMNs. The UE registered with the network needs to be able tobe always reachable by the network. If the UE is in the ECM-CONNECTEDstate (identically the RRC connection state), the network recognizesthat the UE is being provided with service. If the UE is in the ECM-IDLEstate (identically the RRC idle state), however, the situation of the UEis not valid in an eNB, but is stored in the MME. In such a case, onlythe MME is informed of the location of the UE in the ECM-IDLE statethrough the granularity of the list of Tracking Areas (TAs). A single TAis identified by a Tracking Area Identity (TAI) formed of the identifierof a PLMN to which the TA belongs and Tracking Area Code (TAC) thatuniquely expresses the TA within the PLMN.

Thereafter, the UE selects a cell that belongs to cells provided by theselected PLMN and that has signal quality and characteristics on whichthe UE is able to be provided with proper service.

The following is a detailed description of a procedure of selecting acell by a terminal.

When power is turned-on or the terminal is located in a cell, theterminal performs procedures for receiving a service byselecting/reselecting a suitable quality cell.

A terminal in an RRC idle state should prepare to receive a servicethrough the cell by always selecting a suitable quality cell. Forexample, a terminal where power is turned-on just before should selectthe suitable quality cell to be registered in a network. If the terminalin an RRC connection state enters in an RRC idle state, the terminalshould selects a cell for stay in the RRC idle state. In this way, aprocedure of selecting a cell satisfying a certain condition by theterminal in order to be in a service idle state such as the RRC idlestate refers to cell selection. Since the cell selection is performed ina state that a cell in the RRC idle state is not currently determined,it is important to select the cell as rapid as possible. Accordingly, ifthe cell provides a wireless signal quality of a predetermined level orgreater, although the cell does not provide the best wireless signalquality, the cell may be selected during a cell selection procedure ofthe terminal.

A method and a procedure of selecting a cell by a terminal in a 3GPP LTEis described with reference to 3GPP TS 36.304 V8.5.0 (2009 March) “UserEquipment (UE) procedures in idle mode (Release 8)”.

A cell selection process is basically divided into two types.

The first is an initial cell selection process. In this process, UE doesnot have preliminary information about a wireless channel. Accordingly,the UE searches for all wireless channels in order to find out a propercell. The UE searches for the strongest cell in each channel Thereafter,if the UE has only to search for a suitable cell that satisfies a cellselection criterion, the UE selects the corresponding cell.

Next, the UE may select the cell using stored information or usinginformation broadcasted by the cell. Accordingly, cell selection may befast compared to an initial cell selection process. If the UE has onlyto search for a cell that satisfies the cell selection criterion, the UEselects the corresponding cell. If a suitable cell that satisfies thecell selection criterion is not retrieved though such a process, the UEperforms an initial cell selection process.

The cell selection criterion may be defined as below equation 1.Srxlev>0 AND Squal>0where:Srxlev=Q _(rxlevmeas)−(Q _(rxlevmin) +Q _(rxlevminoffset))−PcompensationSqual=Q _(qualmeas)−(Q _(qualmin) +Q _(qualminoffset))  [Equation 1]

Here, the variables in the equation 1 may be defined as below table 1.

TABLE 1 Srxlev Cell selection RX level value (dB) Squal Cell selectionquality value (dB) Q_(rxlevmeas) Measured cell RX level value (RSRP)Q_(qualmeas) Measured cell quality value (RSRQ) Q_(rxlevmin) Minimumrequired RX level in the cell (dBm) Q_(qualmin) Minimum required qualitylevel in the cell (dB) Q_(rxlevminoffset) Offset to the signalledQ_(rxlevmin) taken into account in the Srxlev evaluation as a result ofa periodic search for a higher priority PLMN while camped normally in aVPLMN Q_(qualminoffset) Offset to the signalled Q_(qualmin) taken intoaccount in the Squal evaluation as a result of a periodic search for ahigher priority PLMN while camped normally in a VPLMN Pcompensationmax(P_(EMAX) − P_(PowerClass), 0) (dB) P_(EMAX) Maximum TX power levelan UE may use when transmitting on the uplink in the cell (dBm) definedas PEMAX in [TS 36.101] P_(PowerClass) Maximum RF output power of the UE(dBm) according to the UE power class as defined in [TS 36.101]

Signalled values, i.e., Q_(rxlevminoffset) and Q_(qualminoffset), may beapplied to a case where cell selection is evaluated as a result ofperiodic search for a higher priority PLMN during a UE camps on a normalcell in a VPLMN. During the periodic search for the higher priority PLMNas described above, the UE may perform the cell selection evaluation byusing parameter values stored in other cells of the higher priorityPLMN.

After the UE selects a specific cell through the cell selection process,the intensity or quality of a signal between the UE and a BS may bechanged due to a change in the mobility or wireless environment of theUE. Accordingly, if the quality of the selected cell is deteriorated,the UE may select another cell that provides better quality. If a cellis reselected as described above, the UE selects a cell that providesbetter signal quality than the currently selected cell. Such a processis called cell reselection. In general, a basic object of the cellreselection process is to select a cell that provides UE with the bestquality from a viewpoint of the quality of a radio signal.

In addition to the viewpoint of the quality of a radio signal, a networkmay determine priority corresponding to each frequency, and may informthe UE of the determined priorities. The UE that has received thepriorities preferentially takes into consideration the priorities in acell reselection process compared to a radio signal quality criterion.

As described above, there is a method of selecting or reselecting a cellaccording to the signal characteristics of a wireless environment. Inselecting a cell for reselection when a cell is reselected, thefollowing cell reselection methods may be present according to the RATand frequency characteristics of the cell.

-   -   Intra-frequency cell reselection: UE reselects a cell having the        same center frequency as that of RAT, such as a cell on which        the UE camps on.    -   Inter-frequency cell reselection: UE reselects a cell having a        different center frequency from that of RAT, such as a cell on        which the UE camps on    -   Inter-RAT cell reselection: UE reselects a cell that uses RAT        different from RAT on which the UE camps

The principle of a cell reselection process is as follows.

First, UE measures the quality of a serving cell and neighbor cells forcell reselection.

Second, cell reselection is performed based on a cell reselectioncriterion. The cell reselection criterion has the followingcharacteristics in relation to the measurements of a serving cell andneighbor cells.

Intra-frequency cell reselection is basically based on ranking. Rankingis a task for defining a criterion value for evaluating cell reselectionand numbering cells using criterion values according to the size of thecriterion values. A cell having the best criterion is commonly calledthe best-ranked cell. The cell criterion value is based on the value ofa corresponding cell measured by UE, and may be a value to which afrequency offset or cell offset has been applied, if necessary.

Inter-frequency cell reselection is based on frequency priority providedby a network. UE attempts to camp on a frequency having the highestfrequency priority. A network may provide frequency priority that willbe applied by UEs within a cell in common through broadcastingsignaling, or may provide frequency-specific priority to each UE throughUE-dedicated signaling. A cell reselection priority provided throughbroadcast signaling may refer to a common priority. A cell reselectionpriority for each terminal set by a network may refer to a dedicatedpriority. If receiving the dedicated priority, the terminal may receivea valid time associated with the dedicated priority together. Ifreceiving the dedicated priority, the terminal starts a validity timerset as the received valid time together therewith. While the valid timeris operated, the terminal applies the dedicated priority in the RRC idlemode. If the valid timer is expired, the terminal discards the dedicatedpriority and again applies the common priority.

For the inter-frequency cell reselection, a network may provide UE witha parameter (e.g., a frequency-specific offset) used in cell reselectionfor each frequency.

For the intra-frequency cell reselection or the inter-frequency cellreselection, a network may provide UE with a Neighboring Cell List (NCL)used in cell reselection. The NCL includes a cell-specific parameter(e.g., a cell-specific offset) used in cell reselection.

For the intra-frequency or inter-frequency cell reselection, a networkmay provide UE with a cell reselection black list used in cellreselection. The UE does not perform cell reselection on a cell includedin the black list.

Ranking performed in a cell reselection evaluation process is describedbelow.

A ranking criterion used to apply priority to a cell is defined as inEquation 2.Rs=Qmeas,s+Qhyst,Rn=Qmeas,s−Qoffset  [Equation 2]

In this case, Rs is the ranking criterion of a serving cell, Rn is theranking criterion of a neighbor cell, Qmeas,s is the quality value ofthe serving cell measured by UE, Qmeas,n is the quality value of theneighbor cell measured by UE, Qhyst is the hysteresis value for ranking,and Qoffset is an offset between the two cells.

In Intra-frequency, if UE receives an offset “Qoffsets,n” between aserving cell and a neighbor cell, Qoffset=Qoffsets,n. If UE does notQoffsets,n, Qoffset=0.

In Inter-frequency, if UE receives an offset “Qoffsets,n” for acorresponding cell, Qoffset=Qoffsets,n+Qfrequency. If UE does notreceive “Qoffsets,n”, Qoffset=Qfrequency.

If the ranking criterion Rs of a serving cell and the ranking criterionRn of a neighbor cell are changed in a similar state, ranking priorityis frequency changed as a result of the change, and UE may alternatelyreselect the twos. Qhyst is a parameter that gives hysteresis to cellreselection so that UE is prevented from to alternately reselecting twocells.

UE measures RS of a serving cell and Rn of a neighbor cell according tothe above equation, considers a cell having the greatest rankingcriterion value to be the best-ranked cell, and reselects the cell.

In accordance with the criterion, it may be checked that the quality ofa cell is the most important criterion in cell reselection. If areselected cell is not a suitable cell, UE excludes a correspondingfrequency or a corresponding cell from the subject of cell reselection.

A Radio Link Failure (RLF) is described below.

UE continues to perform measurements in order to maintain the quality ofa radio link with a serving cell from which the UE receives service. TheUE determines whether or not communication is impossible in a currentsituation due to the deterioration of the quality of the radio link withthe serving cell. If communication is almost impossible because thequality of the serving cell is too low, the UE determines the currentsituation to be an RLF.

If the RLF is determined, the UE abandons maintaining communication withthe current serving cell, selects a new cell through cell selection (orcell reselection) procedure, and attempts RRC connectionre-establishment with the new cell.

In the specification of 3GPP LTE, the following examples are taken ascases where normal communication is impossible.

-   -   A case where UE determines that there is a serious problem in        the quality of a downlink communication link (a case where the        quality of a PCell is determined to be low while performing RLM)        based on the radio quality measured results of the PHY layer of        the UE    -   A case where uplink transmission is problematic because a random        access procedure continues to fail in the MAC sublayer.    -   A case where uplink transmission is problematic because uplink        data transmission continues to fail in the RLC sublayer.    -   A case where handover is determined to have failed.    -   A case where a message received by UE does not pass through an        integrity check.

An RRC connection re-establishment procedure is described in more detailbelow.

FIG. 7 is a diagram illustrating an RRC connection re-establishmentprocedure.

Referring to FIG. 7, UE stops using all the radio bearers that have beenconfigured other than a Signaling Radio Bearer (SRB) #0, and initializesa variety of kinds of sublayers of an Access Stratum (AS) (S710).Furthermore, the UE configures each sublayer and the PHY layer as adefault configuration. In this process, the UE maintains the RRCconnection state.

The UE performs a cell selection procedure for performing an RRCconnection reconfiguration procedure (S720). The cell selectionprocedure of the RRC connection re-establishment procedure may beperformed in the same manner as the cell selection procedure that isperformed by the UE in the RRC idle state, although the UE maintains theRRC connection state.

After performing the cell selection procedure, the UE determines whetheror not a corresponding cell is a suitable cell by checking the systeminformation of the corresponding cell (S730). If the selected cell isdetermined to be a suitable E-UTRAN cell, the UE sends an RRC connectionre-establishment request message to the corresponding cell (S740).

Meanwhile, if the selected cell is determined to be a cell that uses RATdifferent from that of the E-UTRAN through the cell selection procedurefor performing the RRC connection re-establishment procedure, the UEstops the RRC connection re-establishment procedure and enters the RRCidle state (S750).

The UE may be implemented to finish checking whether the selected cellis a suitable cell through the cell selection procedure and thereception of the system information of the selected cell. To this end,the UE may drive a timer when the RRC connection re-establishmentprocedure is started. The timer may be stopped if it is determined thatthe UE has selected a suitable cell. If the timer expires, the UE mayconsider that the RRC connection re-establishment procedure has failed,and may enter the RRC idle state. Such a timer is hereinafter called anRLF timer. In LTE spec TS 36.331, a timer named “T311” may be used as anRLF timer. The UE may obtain the set value of the timer from the systeminformation of the serving cell.

If an RRC connection re-establishment request message is received fromthe UE and the request is accepted, a cell sends an RRC connectionre-establishment message to the UE.

The UE that has received the RRC connection re-establishment messagefrom the cell reconfigures a PDCP sublayer and an RLC sublayer with anSRB1. Furthermore, the UE calculates various key values related tosecurity setting, and reconfigures a PDCP sublayer responsible forsecurity as the newly calculated security key values. Accordingly, theSRB 1 between the UE and the cell is open, and the UE and the cell mayexchange RRC control messages. The UE completes the restart of the SRB1,and sends an RRC connection re-establishment complete message indicativeof that the RRC connection re-establishment procedure has been completedto the cell (S760).

In contrast, if the RRC connection re-establishment request message isreceived from the UE and the request is not accepted, the cell sends anRRC connection re-establishment reject message to the UE.

If the RRC connection re-establishment procedure is successfullyperformed, the cell and the UE perform an RRC connection reconfigurationprocedure. Accordingly, the UE recovers the state prior to the executionof the RRC connection re-establishment procedure, and the continuity ofservice is guaranteed to the upmost.

FIG. 8 illustrates substates which may be owned by UE in the RRC_IDLEstate and a substate transition process.

Referring to FIG. 8, UE performs an initial cell selection process(S801). The initial cell selection process may be performed when thereis no cell information stored with respect to a PLMN or if a suitablecell is not discovered.

If a suitable cell is unable to be discovered in the initial cellselection process, the UE transits to any cell selection state (S802).The any cell selection state is the state in which the UE has not campedon a suitable cell and an acceptable cell and is the state in which theUE attempts to discover an acceptable cell of a specific PLMN on whichthe UE may camp. If the UE has not discovered any cell on which it maycamp, the UE continues to stay in the any cell selection state until itdiscovers an acceptable cell.

If a suitable cell is discovered in the initial cell selection process,the UE transits to a normal camp state (S803). The normal camp staterefers to the state in which the UE has camped on the suitable cell. Inthis state, the UE may select and monitor a paging channel based oninformation provided through system information and may perform anevaluation process for cell reselection.

If a cell reselection evaluation process (S804) is caused in the normalcamp state (S803), the UE performs a cell reselection evaluation process(S804). If a suitable cell is discovered in the cell reselectionevaluation process (S804), the UE transits to the normal camp state(S803) again.

If an acceptable cell is discovered in the any cell selection state(S802), the UE transmits to any cell camp state (S805). The any cellcamp state is the state in which the UE has camped on the acceptablecell.

In the any cell camp state (S805), the UE may select and monitor apaging channel based on information provided through system informationand may perform the evaluation process (S806) for cell reselection. Ifan acceptable cell is not discovered in the evaluation process (S806)for cell reselection, the UE transits to the any cell selection state(S802).

Now, a device-to-device (D2D) operation is described. In 3GPP LTE-A, aservice related to the D2D operation is called a proximity service(ProSe). Now, the ProSe is described. Hereinafter, the ProSe is the sameconcept as the D2D operation, and the ProSe and the D2D operation may beused without distinction.

The ProSe includes ProSe direction communication and ProSe directdiscovery. The ProSe direct communication is communication performedbetween two or more proximate UEs. The UEs may perform communication byusing a protocol of a user plane. A ProSe-enabled UE implies a UEsupporting a procedure related to a requirement of the ProSe. Unlessotherwise specified, the ProSe-enabled UE includes both of a publicsafety UE and a non-public safety UE. The public safety UE is a UEsupporting both of a function specified for a public safety and a ProSeprocedure, and the non-public safety UE is a UE supporting the ProSeprocedure and not supporting the function specified for the publicsafety.

ProSe direct discovery is a process for discovering anotherProSe-enabled UE adjacent to ProSe-enabled UE. In this case, only thecapabilities of the two types of ProSe-enabled UE are used. EPC-levelProSe discovery means a process for determining, by an EPC, whether thetwo types of ProSe-enabled UE are in proximity and notifying the twotypes of ProSe-enabled UE of the proximity.

Hereinafter, for convenience, the ProSe direct communication may bereferred to as D2D communication, and the ProSe direct discovery may bereferred to as D2D discovery.

FIG. 9 shows a basic structure for ProSe.

Referring to FIG. 9, the basic structure for ProSe includes an E-UTRAN,an EPC, a plurality of types of UE including a ProSe applicationprogram, a ProSe application server (a ProSe APP server), and a ProSefunction.

The EPC represents an E-UTRAN core network configuration. The EPC mayinclude an MME, an S-GW, a P-GW, a policy and charging rules function(PCRF), a home subscriber server (HSS) and so on.

The ProSe APP server is a user of a ProSe capability for producing anapplication function. The ProSe APP server may communicate with anapplication program within UE. The application program within UE may usea ProSe capability for producing an application function.

The ProSe function may include at least one of the followings, but isnot necessarily limited thereto.

-   -   Interworking via a reference point toward the 3rd party        applications    -   Authorization and configuration of UE for discovery and direct        communication    -   Enable the functionality of EPC level ProSe discovery    -   ProSe related new subscriber data and handling of data storage,        and also handling of the ProSe identities    -   Security related functionality    -   Provide control towards the EPC for policy related functionality    -   Provide functionality for charging (via or outside of the EPC,        e.g., offline charging)

A reference point and a reference interface in the basic structure forProSe are described below.

-   -   PC1: a reference point between the ProSe application program        within the UE and the ProSe application program within the ProSe        APP server. This is used to define signaling requirements in an        application dimension.    -   PC2: a reference point between the ProSe APP server and the        ProSe function. This is used to define an interaction between        the ProSe APP server and the ProSe function. The update of        application data in the ProSe database of the ProSe function may        be an example of the interaction.    -   PC3: a reference point between the UE and the ProSe function.        This is used to define an interaction between the UE and the        ProSe function. A configuration for ProSe discovery and        communication may be an example of the interaction.    -   PC4: a reference point between the EPC and the ProSe function.        This is used to define an interaction between the EPC and the        ProSe function. The interaction may illustrate the time when a        path for 1:1 communication between types of UE is set up or the        time when ProSe service for real-time session management or        mobility management is authenticated.    -   PC5: a reference point used for using control/user plane for        discovery and communication, relay, and 1:1 communication        between types of UE.    -   PC6: a reference point for using a function, such as ProSe        discovery, between users belonging to different PLMNs.    -   SGi: this may be used to exchange application data and types of        application dimension control information.

<ProSe Direct Communication>

ProSe direct communication is communication mode in which two types ofpublic safety UE can perform direct communication through a PC 5interface. Such communication mode may be supported when UE is suppliedwith services within coverage of an E-UTRAN or when UE deviates fromcoverage of an E-UTRAN.

FIG. 10 shows the deployment examples of types of UE performing ProSedirect communication and cell coverage.

Referring to FIG. 10(a), types of UE A and B may be placed outside cellcoverage. Referring to FIG. 10(b), UE A may be placed within cellcoverage, and UE B may be placed outside cell coverage. Referring toFIG. 10(c), types of UE A and B may be placed within single cellcoverage. Referring to FIG. 10(d), UE A may be placed within coverage ofa first cell, and UE B may be placed within coverage of a second cell.

ProSe direct communication may be performed between types of UE placedat various positions as in FIG. 10.

Meanwhile, the following IDs may be used in ProSe direct communication.

A source layer-2 ID: this ID identifies the sender of a packet in the PC5 interface.

A destination layer-2 ID: this ID identifies the target of a packet inthe PC 5 interface.

An SA L1 ID: this ID is the ID of scheduling assignment (SA) in the PC 5interface.

FIG. 11 shows a user plane protocol stack for ProSe directcommunication.

Referring to FIG. 11, the PC 5 interface includes a PDCH, RLC, MAC, andPHY layers.

In ProSe direct communication, HARQ feedback may not be present. An MACheader may include a source layer-2 ID and a destination layer-2 ID.

<Radio Resource Assignment for ProSe Direct Communication>

ProSe-enabled UE may use the following two types of mode for resourceassignment for ProSe direct communication.

1. Mode 1

Mode 1 is mode in which resources for ProSe direct communication arescheduled by an eNB. UE needs to be in the RRC_CONNECTED state in orderto send data in accordance with mode 1. The UE requests a transmissionresource from an eNB. The eNB performs scheduling assignment andschedules resources for sending data. The UE may send a schedulingrequest to the eNB and send a ProSe Buffer Status Report (BSR). The eNBhas data to be subjected to ProSe direct communication by the UE basedon the ProSe BSR and determines that a resource for transmission isrequired.

2. Mode 2

Mode 2 is mode in which UE directly selects a resource. UE directlyselects a resource for ProSe direct communication in a resource pool.The resource pool may be configured by a network or may have beenpreviously determined.

Meanwhile, if UE has a serving cell, that is, if the UE is in theRRC_CONNECTED state with an eNB or is placed in a specific cell in theRRC_IDLE state, the UE is considered to be placed within coverage of theeNB.

If UE is placed outside coverage, only mode 2 may be applied. If the UEis placed within the coverage, the UE may use mode 1 or mode 2 dependingon the configuration of an eNB.

If another exception condition is not present, only when an eNB performsa configuration, UE may change mode from mode 1 to mode 2 or from mode 2to mode 1.

<ProSe Direct Discovery>

ProSe direct discovery refers to a procedure that is used forProSe-enabled UE to discover another ProSe-enabled UE in proximity andis also called D2D direct discovery. In this case, E-UTRA radio signalsthrough the PC 5 interface may be used. Information used in ProSe directdiscovery is hereinafter called discovery information.

FIG. 12 shows the PC 5 interface for D2D direct discovery.

Referring to FIG. 12, the PC 5 interface includes an MAC layer, a PHYlayer, and a ProSe Protocol layer, that is, a higher layer. The higherlayer (the ProSe Protocol) handles the permission of the announcementand monitoring of discovery information. The contents of the discoveryinformation are transparent to an access stratum (AS). The ProSeProtocol transfers only valid discovery information to the AS forannouncement.

The MAC layer receives discovery information from the higher layer (theProSe Protocol). An IP layer is not used to send discovery information.The MAC layer determines a resource used to announce discoveryinformation received from the higher layer. The MAC layer produces anMAC protocol data unit (PDU) for carrying discovery information andsends the MAC PDU to the physical layer. An MAC header is not added.

In order to announce discovery information, there are two types ofresource assignment.

1. Type 1

The type 1 is a method for assigning a resource for announcing discoveryinformation in a UE-not-specific manner. An eNB provides a resource poolconfiguration for discovery information announcement to types of UE. Theconfiguration may be signaled through the SIB.

UE autonomously selects a resource from an indicated resource pool andannounces discovery information using the selected resource. The UE mayannounce the discovery information through a randomly selected resourceduring each discovery period.

2. Type 2

The type 2 is a method for assigning a resource for announcing discoveryinformation in a UE-specific manner. UE in the RRC_CONNECTED state mayrequest a resource for discovery signal announcement from an eNB throughan RRC signal. The eNB may announce a resource for discovery signalannouncement through an RRC signal. A resource for discovery signalmonitoring may be assigned within a resource pool configured for typesof UE.

An eNB 1) may announce a type 1 resource pool for discovery signalannouncement to UE in the RRC_IDLE state through the SIB. Types of UEwhose ProSe direct discovery has been permitted use the type 1 resourcepool for discovery information announcement in the RRC_IDLE state.Alternatively, the eNB 2) announces that the eNB supports ProSe directdiscovery through the SIB, but may not provide a resource for discoveryinformation announcement. In this case, UE needs to enter theRRC_CONNECTED state for discovery information announcement.

An eNB may configure that UE has to use a type 1 resource pool fordiscovery information announcement or has to use a type 2 resourcethrough an RRC signal in relation to UE in the RRC_CONNECTED state.

FIG. 13 is an embodiment of a ProSe discovery process.

Referring to FIG. 13, it is assumed that UE A and UE B haveProSe-enabled application programs managed therein and have beenconfigured to have a ‘friend’ relation between them in the applicationprograms, that is, a relationship in which D2D communication may bepermitted between them. Hereinafter, the UE B may be represented as a‘friend’ of the UE A. The application program may be, for example, asocial networking program. ‘3GPP Layers’ correspond to the functions ofan application program for using ProSe discovery service, which havebeen defined by 3GPP.

Direct discovery between the types of UE A and B may experience thefollowing process.

1. First, the UE A performs regular application layer communication withthe APP server. The communication is based on an Application ProgramInterface (API).

2. The ProSe-enabled application program of the UE A receives a list ofapplication layer IDs having a ‘friend’ relation. In general, theapplication layer ID may have a network access ID form. For example, theapplication layer ID of the UE A may have a form, such as“adam@example.com.”

3. The UE A requests private expressions code for the user of the UE Aand private representation code for a friend of the user.

4. The 3GPP layers send a representation code request to the ProSeserver.

5. The ProSe server maps the application layer IDs, provided by anoperator or a third party APP server, to the private representationcode. For example, an application layer ID, such as adam@example.com,may be mapped to private representation code, such as “GTER543#2FSJ67DFSF.” Such mapping may be performed based on parameters (e.g., amapping algorithm, a key value and so on) received from the APP serverof a network.

6. The ProSe server sends the types of derived representation code tothe 3GPP layers. The 3GPP layers announce the successful reception ofthe types of representation code for the requested application layer IDto the ProSe-enabled application program. Furthermore, the 3GPP layersgenerate a mapping table between the application layer ID and the typesof representation code.

7. The ProSe-enabled application program requests the 3GPP layers tostart a discovery procedure. That is, the ProSe-enabled applicationprogram requests the 3GPP layers to start discovery when one of provided‘friends’ is placed in proximity to the UE A and direct communication ispossible. The 3GPP layers announces the private representation code(i.e., in the above example, “GTER543 #2FSJ67DFSF”, that is, the privaterepresentation code of adam@example.com) of the UE A. This ishereinafter called ‘announcement’. Mapping between the application layerID of the corresponding application program and the privaterepresentation code may be known to only ‘friends’ which have previouslyreceived such a mapping relation, and the ‘friends’ may perform suchmapping.

8. It is assumed that the UE B operates the same ProSe-enabledapplication program as the UE A and has executed the aforementioned 3 to6 steps. The 3GPP layers placed in the UE B may execute ProSe discovery.

9. When the UE B receives the aforementioned ‘announce’ from the UE A,the UE B determines whether the private representation code included inthe ‘announce’ is known to the UE B and whether the privaterepresentation code is mapped to the application layer ID. As describedthe 8 step, since the UE B has also executed the 3 to 6 steps, it isaware of the private representation code, mapping between the privaterepresentation code and the application layer ID, and correspondingapplication program of the UE A. Accordingly, the UE B may discover theUE A from the ‘announce’ of the UE A. The 3GPP layers announce thatadam@example.com has been discovered to the ProSe-enabled applicationprogram within the UE B.

In FIG. 13, the discovery procedure has been described by taking intoconsideration all of the types of UE A and B, the ProSe server, the APPserver and so on. From the viewpoint of the operation between the typesof UE A and B, the UE A sends (this process may be called announcement)a signal called announcement, and the UE B receives the announce anddiscovers the UE A. That is, from the aspect that an operation thatbelongs to operations performed by types of UE and that is directlyrelated to another UE is only step, the discovery process of FIG. 13 mayalso be called a single step discovery procedure.

FIG. 14 is another embodiment of a ProSe discovery process.

In FIG. 14, types of UE 1 to 4 are assumed to types of UE included inspecific group communication system enablers (GCSE) group. It is assumedthat the UE 1 is a discoverer and the types of UE 2, 3, and 4 arediscoveree. UE 5 is UE not related to the discovery process.

The UE 1 and the UE 2-4 may perform a next operation in the discoveryprocess.

First, the UE 1 broadcasts a target discovery request message (may behereinafter abbreviated as a discovery request message or M1) in orderto discover whether specific UE included in the GCSE group is inproximity. The target discovery request message may include the uniqueapplication program group ID or layer-2 group ID of the specific GCSEgroup. Furthermore, the target discovery request message may include theunique ID, that is, application program private ID of the UE 1. Thetarget discovery request message may be received by the types of UE 2,3, 4, and 5.

The UE 5 sends no response message. In contrast, the types of UE 2, 3,and 4 included in the GCSE group send a target discovery responsemessage (may be hereinafter abbreviated as a discovery response messageor M2) as a response to the target discovery request message. The targetdiscovery response message may include the unique application programprivate ID of UE sending the message.

An operation between types of UE in the ProSe discovery processdescribed with reference to FIG. 14 is described below. The discoverer(the UE 1) sends a target discovery request message and receives atarget discovery response message, that is, a response to the targetdiscovery request message. Furthermore, when the discoveree (e.g., theUE 2) receives the target discovery request message, it sends a targetdiscovery response message, that is, a response to the target discoveryrequest message. Accordingly, each of the types of UE performs theoperation of the 2 step. In this aspect, the ProSe discovery process ofFIG. 14 may be called a 2-step discovery procedure.

In addition to the discovery procedure described in FIG. 14, if the UE 1(the discoverer) sends a discovery conform message (may be hereinafterabbreviated as an M3), that is, a response to the target discoveryresponse message, this may be called a 3-step discovery procedure.

The present invention will now be described.

As described above, the UE supporting D2D operation may operate in anyone of two modes with respect to resource allocation.

First, mode 1 is a mode in which an eNB schedules a resource for a D2Doperation, and the UE must be in an RRC connection mode in order totransmit D2D data. The UE may request the D2D transmission resource toan eNB and the eNB may schedule the transmission resources forscheduling assignment and data transmission. The UE may transmit a BSRand a scheduling request (DSR) to the eNB. The BSR notifies the UE ofdata by D2D communication and at the same time allows the BS to estimatethe resources required for the data transmission.

Mode 2 is a mode in which the UE itself selects a resource for D2Doperation and selects resources for transmitting D2D data and schedulingallocation or the like in a configured resource pool.

For a UE in the RRC idle state, the eNB may 1) provide a transmissionresource pool according to the mode 2 through an SIB (system informationblock). In this case, the UE may perform D2D communication using themode 2 transmission resource pool in the RRC idle state. Or the eNBinforms that it supports D2D operation and may not provide aconfiguration for resources for D2D communication in the SIB. In thiscase, the UE may not perform D2D communication in the RRC idle state,and may perform D2D communication after entering the RRC connectionstate.

The UE in the RRC connection state may inform the eNB that it wants toperform transmission according to D2D communication. The eNB maydetermine whether D2D communication of the UE isallowed/permitted/authorized using the context for the UE received fromthe MME.

The eNB may configure a transmission resource pool according to the mode2 through a dedicated signal for the UE in the RRC connection state. TheUE may use the transmission resource pool according to the mode 2 whileit is in the RRC connection state without any specific limitation.Alternatively, the eNB may configure a resource pool according to themode 2 that may be used only in an exceptional case through a dedicatedsignal for the UE in the RRC connection state. In the case of other thanthe exceptional case, the UE may perform D2D communication using themode 1, i.e., resources allocated by the eNB.

Meanwhile, the UE may leave the RRC connection state for variousreasons. For example, the UE leaves the RRC connection state when theeNB releases the RRC connection to the UE or if the RRC connectionre-establishment procedure fails. The RRC connection re-establishmentprocedure may be regarded as a failure if T311 expires or a cell ofanother RAT is selected.

When the UE leaves the RRC connection state, it may perform thefollowing operations.

For example, when leaving the RRC connection state, the UE 1) resets theMAC and 2) stops all timers running among the remaining timers exceptT320, T325, and T330. 3) Also, the UE releases all radio resources. TheUE performs release of the RLC entity, release of the MAC configuration,release of the associated PDCP entity, etc. for all established radiobearers (RBs).

That is, when the UE leaves the RRC connection state, the UE releasesall radio resource configurations including the D2D configuration.Therefore, if the UE performing the D2D operation leaves the RRCconnection state, the continuity of the D2D operation may not beguaranteed.

In order to solve such a problem, in the present invention, it isproposed that, when a UE leaving the RRC connection state releases allradio resources, the entity associated with D2D communication and theconfiguration associated with D2D communication are excluded. That is,when the UE receives the D2D configuration on the D2D operation from thenetwork and leaves the radio resource control (RRC) connection state,the UE may maintain, without releasing, the D2D configuration on the D2Doperation.

FIG. 15 shows a method of managing a configuration on the D2D operationof UE according to an embodiment of the present invention. The UE may bein a state that the D2D configuration associated with the D2D operationhas already received from the eNB.

Referring to FIG. 15, the UE determines whether a transition conditionfrom an RRC connection state to another state is satisfied (S210).

For example, the condition may be that the eNB has released the RRCconnection to the UE or the UE has performed the RRC connectionre-establishment procedure but failed to re-establish the RRCconnection.

If the condition is satisfied and thus the UE leaves the RRC connectionstate, the UE does not release but maintain the D2D configuration on theD2D operation (S220).

The D2D configuration may be provided via a dedicated signal to the UEor via system information block (SIB) to be broadcasted.

The following table shows an example of a SIB (which may also bereferred to as ProSe system information) that includes the D2Dconfiguration for D2D operation.

TABLE 2 -- ASN1START SystemInformationBlockType18-r12 ::= SEQUENCE { commConfig-r12 SEQUENCE {   frequncyIndicator SL-FrequencyIndication-r12   commRxPool-r12   SL-CommRxPoolList-r12,  commTxPoolNormalCommon-r12   SL-CommTxPoolList-r12   OPTIONAL, -- NeedOR   commTxPoolExceptional-r12   SL-CommTxPoolList-r12   OPTIONAL, --Need OR   commSyncConfig-r12   SL-SyncConfigList-r12  OPTIONAL -- NeedOR  }    OPTIONAL, -- Need OR  lateNonCriticalExtension   OCTET STRING   OPTIONAL,  ... } -- ASN1STOP

In the above table, ‘frequencyIndicator’ is a frequency indicatorindicating the frequency to which ‘commConfig’ applies. ‘CommRxPool’ in‘commConfig’ indicates resources allowed to receive signals on D2Dcommunication in RRC idle state and RRC connection state.‘CommTxPoolNormalcommon’ indicates a resource that is allowed totransmit a signal on D2D communication at a frequency other than aspecific frequency in an RRC idle state or an RRC connection state.‘CommTxPoolExceptional’ indicates a resource that is allowed to transmitsignals on D2D communication under exceptional conditions. That is, thenetwork may inform the UE of the D2D resources that may be used for D2Doperation and the frequencies where the D2D resources may be used.

That is, the SIB on the D2D operation may inform at least one of aresource (resource pool) that can be used for transmission of the D2Dsignal usable in the mode 2, and a resource (resource pool) that can beused for receiving the D2D signal.

The UE selects a specific cell and performs a D2D operation according tothe maintained D2D configuration until receiving a SIB related to a D2Doperation from the selected cell (S230). For example, suppose a UE is inRRC connection state with cell 1 and receives a D2D configuration fromcell 1. As an example, the UE may assume that it has received a D2Dconfiguration from the cell 1 via the SIB as shown in Table 2. Asanother example, it may be assumed that the UE has received a D2Dconfiguration similar to Table 2 from the cell 1 via the UE dedicatedsignal. If the UE performed but has failed the RRC connectionre-establishment procedure due to a problem with the radio link with thecell 1, the UE may leave the RRC connection state and enter the RRC idlestate. In this case, the UE maintains the D2D configuration withoutreleasing it. Also, the UE may perform a D2D operation using the D2Dconfiguration received in the cell 1 until it camped on another cell,e.g., the cell 2 and receives a SIB containing a D2D configurationprovided by the cell 2.

In the above example, if cell 1 provides a configuration for the D2Dreceiving resource pool via the SIB or UE dedicated signal, the UE maycontinue receiving D2D under certain conditions even if leaving the RRCconnection state in relation to the cell 1. Also, if the cell 1 providesa configuration for the D2D transmission resource pool through the SIBor the UE dedicated signal, the UE may continue the D2D transmissionunder a certain condition even when leaving the RRC connection state inrelation to the cell.

In the above example, it is also possible to receive a configuration ofa separate D2D configuration from cell 1 that may be applied after theUE leaves the RRC connection state in relation to cell 1. If the UEreceives the separate D2D configuration from the SIB of cell 1 or fromthe cell 1 via the UE-dedicated signal, and leaves the RRC connectionstate, the UE may continue the D2D operation without interruption byusing the separate D2D configuration.

In the above example, it may be necessary to limit the operation ofperforming the D2D operation to a specific condition by applying the D2Dconfiguration after the UE receives the D2D configuration from cell 1and leaves the RRC connection state. For example, it may be desirablethat the operation of using the D2D configuration received by the UE inthe cell 1 even when the UE is not connected to the cell 1 is allowed tobe exceptionally allowed only for a certain amount of time to themaximum extent. To this end, the UE initiates a timer when leaving theRRC connection state, and may perform the D2D operation by applying theD2D configuration during the time indicated by the timer (or limited tothe time indicated by the timer). The time of the timer may beconfigured by the network or it may be a predetermined value.

If the D2D configuration having received from the existing cellcontinues to be used even after a certain time has elapsed since the UEleft the RRC connection state, then it may cause significantinterference to the network and thus the D2D configuration received fromthe existing cell is used for a certain period of time.

After selecting the regular cell (cell 2) in the RRC idle state, the UEmay receive the system information including the D2D configurationapplicable in the RRC idle state through the cell 2. If there is aconfiguration for the mode 2 receiving resource pool in the systeminformation, the UE will be able to receive D2D communication using themode 2 receiving resource pool in the RRC idle state. Further, if thesystem information includes a configuration for the mode 2 transmissionresource pool, the UE may transmit a signal according to the D2Dcommunication using the mode 2 transmission resource pool in the RRCidle state.

According to the method of FIG. 15, the continuity of the D2D operationis guaranteed even if the UE leaves the RRC connection state during theD2D operation. More specifically, the D2D operation may be continueduntil the UE completes the cell selection procedure while leaving theRRC connection state. The D2D operation may be a D2D communication.

Hereinafter, a cell selection will be described when leaving the RRCconnection state.

Upon receiving the RRC connection release message, the UE may change thestate from the RRC connection state to the RRC idle state. In this case,a field ‘redirectedCarrierInfo’ may be included in the RRC connectionrelease message. The ‘redirectedCarrierInfo’ may include a list offrequencies that the UE may attempt to camp on. If the field‘redirectedCarrierInfo’ is included in the RRC connection releasemessage, the UE may find a regular cell to camp on at any one of thefrequencies indicated by the ‘redirectedCarrierInfo’.

However, it may be occurred that the UE is interested in D2Dcommunication and the there is no frequency at which D2D communicationis available among the frequencies indicated by the‘redirectedCarrierInfo’. In this case, UE may ignore‘redirectedCarrierInfo’ and search for another frequency capable of D2Dcommunication. The frequency at which D2D communication is available maybe informed to the UE in advance.

If there is any frequency at which D2D communication is available amongthe frequencies indicated by the ‘redirectedCarrierInfo’, the priorityof the corresponding frequency may be configured to the highestpriority.

Meanwhile, if the UE may not find a regular cell at the frequenciesindicated by ‘redirectedCarrierInfo’, the UE may be allowed to camp onany regular cell of the indicated RAT.

If ‘redirectedCarrierInfo’ is not included in the RRC connection releasemessage, the UE may select a regular cell at the EUTRA carrierfrequency.

If the regular cell is not found in the procedure, the UE may performcell selection using information stored in the UE itself to search for aregular cell to camp on.

If the RRC connection release message includes ‘redirectedCarrierInfo’in the case of returning back to the RRC idle state after returning tothe RRC connection state from the any cell camped on state, then the UEmay try to camp on to find any acceptable cell of the indicated by the‘redirectedCarrierInfo’. If the UE does not find the acceptable cell,the UE may be allowed to camp on any acceptable cell of the indicatedRAT.

If the RRC connection release message does not include‘redirectedCarrierInfo’, the UE may attempt to select an acceptable cellat the EUTRA carrier frequency. If no acceptable cell is found, the UEmay find an acceptable cell of any PLMN in a random cell selectionstate.

In cell selection/reselection for changing a cell, the UE may inform ahigher layer of the UE itself whether or not it may perform D2Dcommunication. The UE may notify the higher layer only when the state ischanged as compared with the previously announced state.

The UE may initiate the RRC connection re-establishment procedure if oneof the following conditions is met:

If 1) the radio link failure is detected, 2) the handover fails, 3)transition from the EUTRA failure, 4) the lower layer reports theintegrity check failure, and 5) the RRC connection reconfiguration failsetc., is occurred, then the re-establishment procedure may be initiated.

FIG. 16 shows a case where the RRC connection re-establishment procedureis successful.

Referring to FIG. 16, the UE transmits an RRC connection reestablishmentrequest (RRCConnectionReestablishmentRequest) message to the EUTRAN(S181). The RRC connection re-establishment procedure may be caused by aradio link failure (RLF) or a handover failure. The RRC connectionre-establishment request message may include information indicating thecell ID, the UE ID, and the cause of re-establishment where the RLF, thehandover failure or the like is occurred.

The network transmits an RRC connection reestablishment(RRCConnectionReestablishment) message to the UE (S182).

If the RRC connection re-establishment procedure is successfullyperformed, the UE transmits an RRC connection re-establishmentcompletion (RRCConnectionReestablishmentComplete) message (S183).

FIG. 17 shows a case where the RRC connection re-establishment procedurefails.

Referring to FIG. 17, the UE transmits an RRC connectionre-establishment request message to the network (S191).

The network transmits an RRC connection reestablishment rejection(RRConnectionReestablishmentReject) message to the UE (S192).

When the UE initiates the RRC connection re-establishment procedure, itmay perform the operation as shown in the following table.

TABLE 3 1> stop timer T310, if running; 1> stop timer T312, if running;1> start timer T311; 1> suspend all RBs except SRB0; 1> reset MAC; 1>release the SCell(s), if configured, in accordance with 5.3.10.3a; 1>apply the default physical channel configuration as specified in 9.2.4;1> apply the default semi-persistent scheduling configuration asspecified in 9.2.3: 1> apply the default MAC main configuration asspecified in 9.2.2; 1> release powerPrefIndicationConfig, if configuredand stop timer T340, if running: 1> release reportProximityConfig andclear any associated proximity status reporting timer; 1> releaseobtainLocationConfig, if configured: 1> release ide-Config, ifconfigured; 1> release measSubframePatternPCell, if configured; 1> ifconnected as an RN and configured with an RN subframe configuration:  2>release the RN subframe configuration; 1> perform cell selection inaccordance with the cell selection process as specified in TS 36.304[4]; 1> Keep the configuration for D2D communication.

The differences distinctive from the prior art in the table is that theUE maintains the configuration for D2D (e.g., D2D communication)operation (“Keep the configuration for D2D Communication”) in the RRCconnection re-establishment process as shown at the last part.

If the remaining operations may be performed in the same manner suchthat if T310 is in operation, then T310 is stopped, while if T312 is inoperation, then T312 is stopped, T311 is started, all RBs except SRB0are stopped, and MAC is reset, and if the secondary cell has beenconfigured, the secondary cell is released, a default physical channelconfiguration is applied, a default semi-static scheduling configurationis applied, and a default MAC main configuration is applied,‘powerPrefIndicationConfig’, ‘reportProximityConfig’,‘obtatinLocationConfig’, ‘idc-Config’, ‘measSubframePatternPCell’ or thelike is released, and if there is a connection with the relay stationand there is a subframe configuration for the connection, then thesubframe configuration is released, and the cell reselection isperformed.

The above T310, 311, 312 is as follows.

TABLE 4 Timer Start Stop At expiry T310 Upon detecting physical Uponreceiving N311 If security is not activated: go to layer problems forthe consecutive in-sync indications RRC_IDLE else: initiate the PCel)from lower layers for the PCell, connection re-establishment upontriggering the handover procedure procedure and upon initiating theconnection re-establishment procedure T311 Upon initiating the RRCSelection of a suitable E-UTRA Enter RRC_IDLE state connection re- cellor a cell using another RAT establishment procedure T312 Upon triggeringa Upon receiving N311 If security is not activated: go to measurementreport for consecutive in-sync indications RRC_IDLE else: initiate the ameasurement identity from lower layers, upon connection re-establishmentfor which T312 has triggering the handover procedure been configured,while procedure, upon initiating the T310 is running connectionre-establishment procedure, and upon the expiry of T310

Hereinafter, a cell selection/reselection process of a UE will bedescribed by considering system information on D2D operation.

The UE interested in D2D communication may consider a configuration forD2D communication, e.g., SIB 18 in Table 2 as essential systeminformation. Therefore, when the UE interested in D2D communication at aspecific frequency attempts to camp on a cell of the carrier of thespecific frequency, considers the system information including theconfiguration on the D2D communication as the essential systeminformation.

FIG. 18 shows an example of a serving frequency and a frequency forperforming D2D operation on a UE-by-UE basis.

Referring to FIG. 18, UE 1 has f1 frequency as the serving frequency andf2 frequency as the ProSe frequency for performing the D2D operation. UE2 has f2 frequency as both the serving frequency and the ProSe frequencyfor performing the D2D operation. UE 3 has the f1 frequency as both theserving frequency and the ProSe frequency for performing the D2Doperation.

FIG. 19 illustrates a cell selection method of a UE according to anembodiment of the present invention.

Referring to FIG. 19, the UE determines whether a specific cell providessystem information including a configuration on a D2D operation (S310).

If the specific cell does not provide the system information, the UEbars the specific cell (S320). Barring the specific cell means that theUE excludes the specific cell from the RRC connection target cell, andas a result, the UE is excluded from the cell selection target.

The UE continues to select a cell that is not the specific cell (S330).

The UE finds that the cell that has attempted to camp-on in the RRC idlestate does not provide the system information including theconfiguration on the D2D operation (e.g., as described in the abovetable 2, such system information may be referred to as ProSe systeminformation), then the cell is barred, and another cell may be found.

For example, if the UE finds that the cell that is attempted to becamped on in the RRC idle state does not support D2D communication atall, then it may bar the cell and find another cell.

If the cell in the RRC connection state does not support the D2Dcommunication through the RRC connection re-establishment procedure, theUE releases the RRC connection and enters the RRC idle state. Or the UEmay ignore the cell or consider that the cell is barred, and it maycontinue the cell selection procedure. Whether the D2D communication issupported or not, may be indicated by SIB 1 or SIB 2. For example, itmay be indicated through a separate indicator indicating whether the D2Dcommunication is supported or not. As another example, a D2Dcommunication configuration may be indicated through which the systeminformation (e.g., SIB 18) is scheduled or not.

If the UE, which is the RRC connection state, identifies that thespecific cell that the UE tried to connect does not provide ProSe systeminformation, the UE may release the RRC connection and enter the RRCidle state. Or, the UE may ignore the cell or consider that the cell isbarred and continue the cell selection procedure.

If the UE moves to the RRC idle state, the UE may perform priorityhandling by itself. The basic principle of priority handling of the UEitself is to consider the frequency that the UE is interested in ashaving the highest priority under a certain condition.

For example, in the case of a closed subscriber group (CSG), the certaincondition is that the UE must be a CSG member of a cell. In case ofMBMS, the certain condition is the capability of the UE and the MBMSrelated SIB broadcast state. The condition definition is important interms of 1) limiting the application of priority handling by the UEitself and 2) ensuring continuity of the service of interest.

The same principle may be applied to reselection of the D2D frequency.If the UE is interested in D2D communication and should camp on thefrequency at which D2D communication is to be performed and thus is ableto perform D2D communication, the UE may consider the frequency as thehighest priority frequency during the D2D communication session.

According to the method of FIG. 19, a cell not providing ProSe systeminformation is not selected in the cell selection/reselection process ofthe UE. That is, only the cell providing the ProSe system information isselected in the cell selection/reselection process of the UE. If thecell providing the ProSe system information in the cellselection/reselection process is selected within the acceptable time, noproblem in the continuity of the D2D operation is occurred. Even if thecell providing the ProSe system information is not selected within apredetermined time and leaves the RRC connection state, the UE maintainsthe D2D configuration (D2D configuration having provided by the ProSesystem information of the existing cell) having received from theexisting cell, and thus the D2D operation may be performed using the D2Dconfiguration.

FIG. 20 is a block diagram showing a UE in which an embodiment of thepresent invention is implemented.

Referring to FIG. 20, a UE 1100 includes a processor 1110, a memory1120, and a radio frequency unit (RF) unit 1130. Processor 1110implements the proposed functionality, process and/or method. Forexample, the processor 1110 may receive a D2D configuration on the D2Doperation from a network, and maintaining, without releasing, the D2Dconfiguration on the D2D operation, when leaving a RRC (radio resourcecontrol) connection state. For example, the processor 1110 determineswhether a condition to transit from the RRC connection state to anotherstate is satisfied, and may maintain, without releasing, the D2Dconfiguration provided in system information on the D2D operation, whenthe condition is satisfied and thus leaving the RRC connection state.

The RF unit 1130 is connected to the processor 1110 and sends andreceives radio signals.

The processor may include Application-Specific Integrated Circuits(ASICs), other chipsets, logic circuits, and/or data processors. Thememory may include Read-Only Memory (ROM), Random Access Memory (RAM),flash memory, memory cards, storage media and/or other storage devices.The RF unit may include a baseband circuit for processing a radiosignal. When the above-described embodiment is implemented in software,the above-described scheme may be implemented using a module (process orfunction) which performs the above function. The module may be stored inthe memory and executed by the processor. The memory may be disposed tothe processor internally or externally and connected to the processorusing a variety of well-known means.

What is claimed is:
 1. A method for managing a configuration for device-to-device (D2D) operation of a user equipment (UE) in a wireless communication system, the method comprising: receiving a D2D configuration on the D2D operation from a network; maintaining, without releasing, the D2D configuration on the D2D operation, when the UE leaves a RRC (radio resource control) connection state; and performing the D2D operation based on the D2D configuration on the D2D operation, after the UE leaves the RRC connection state, wherein when the UE leaves the RRC connection state, the UE releases all radio resources except radio resources for the D2D operation.
 2. The method of claim 1, further comprising: determining whether a condition to transit from the RRC connection state to another state is satisfied, wherein the condition is characterized in that the network releases the RRC connection on the UE, or the UE fails a RRC connection reconfiguration with the network.
 3. The method of claim 1, wherein the D2D operation is performed by applying the maintained D2D configuration until the UE selects a specific cell and receives system information on a D2D operation from the selected specific cell.
 4. The method of claim 3, wherein a cell which does not provide system information on a D2D operation is barred, in selecting the specific cell.
 5. The method of claim 3, wherein the specific cell is selected from cells which provide system information on a D2D operation.
 6. The method of claim 1, wherein when the UE leaves a RRC connection state, a timer is started, and the UE performs a D2D operation by applying the D2D configuration only for a time duration indicated by the timer.
 7. The method of claim 1, wherein the network provides an additional D2D configuration other than the D2D configuration, and the additional D2D configuration is available to use after the UE releases a RRC connection state.
 8. The method of claim 1, wherein the D2D operation is a D2D communication.
 9. The method of claim 1, wherein the UE maintains the D2D configuration on the D2D operation in a RRC connection re-establishment process.
 10. A user equipment (UE) comprising: a transmitter and receiver; and a processor operatively connected to the RF unit, and configured to be operated, and wherein the processor is configured to: control the receiver to receive a D2D configuration on the D2D operation from a network; maintain, without releasing, the D2D configuration on the D2D operation, when the UE leaves a RRC (radio resource control) connection state, perform the D2D operation based on the D2D configuration on the D2D operation, after the UE leaves the RRC connection state, wherein when the UE leaves the RRC connection state, the UE releases all radio resources except radio resources for the D2D operation. 